Hepatitis B virus (HBV) caused acute and chronic hepatitis and is a major cause of liver cancer. Clearance of HBV is thought to be mediated by the MHC Class I restricted cytotoxic T lymphocyte (CTL) response. Systematically administered HBsAg CTL, as well as IL-2 and TNF alpha, can noncytopathically downregulate HBV RNA expression in the liver of HBsAg positive mice at the post-transcriptional level. The goal of this project is to determine the molecular basis for this antiviral effect by testing the hypothesis that soluble products of HBV specific CTL activate one or more hepatocellular effector protein(s) that cause the specific destabilization and/or the active degradation of HBV transcripts. The specific aims of this project are: 1) to determine if the cytokine or CTL induced downregulation affects all of the HBV transcripts at the post- transcriptional level: 2) to define the target sequence(s) on the HBV S transcript that is (are) important for destabilization/degradation following cytokine or CTL administration in HBV transgenic mice; and, 3) to define the intracellular effector molecules that cause the specific reduction in HBV RNA. The target sequence will be defined by testing HBV deletion mutants for the negative regulatory effect by in vitro decay assays and in vivo using adenovirus vectors. A search will be made for hepatocellular proteins that recognize and bind the target sequence using RNA-protein binding assays and by screening of a cDNA expression library. Elucidation of the mechanisms that negatively regulate HBV RNA expression may contribute to our understanding of the pathways that lead to viral clearance and/or persistence and may be helpful in the development of antiviral drugs.